With Xcode 8 xcodebuild uses Swift 3 to build by default. This will be great in about six months, but right now it’s somewhat of a pain. I am using Carthage as my dependency manager and all of those are using Swift 2.x still.

If you want to build your Carthage dependencies (or anything from the command line for that matter) using Swift 2.3, add the TOOLCHAINS parameter.

TOOLCHAINS=com.apple.dt.toolchain.Swift_2_3 carthage build

Update: As of Carthage 0.17 you can use a --toolchain option. This doesn’t help with raw xcodebuild obviously.